In conventional machine tools, such as lathes, drill presses, milling machines, cutters, grinders, gear forming or inspection equipment, etc., a workpiece or tool must be gripped securely and rotated precisely in alignment with a turning axis of the machine tool. For a workpiece or tool with a bore or center hole of a given inside diameter, an arbor typically has a collet as an outward expansion element which is pressed radially outwardly against the internal sides of the bore or center hole by the inclined faces of an arbor body movable axially relative to the expansion element. A desired objective for such arbor is to be able to expand and release the expansion element to hold a given inside or outside diameter uniformly and with a minimum of offset or distortion, such that the workpiece or tool is repeatedly gripped by the expansion pressure as precisely evenly as possible and rotated with its center axis in precise alignment with the machine tool axis. In conventional assemblies, a special chucking tool is required to open or close the arbor, requiring the operator to keep the chucking tool on the bench within reach at all times. This can be inconvenient for an operator who is busy handling a difficult to maneuver workpiece or has a busy bench. Therefore, it is a desired object of the present invention to provide an arbor for such machine tool that allows chucking without using a tool.